Hmm if I were you, I will upgrade the headphones or iem first instead of adding amp. Headphone or iem wise, still have room to upgrade.
Not sure if you know, c3 doesn't have line out. Adding amp is double amping. Though sometimes double amping might also have good effects but I must say you must meet the "coincidence". Double amping is not the usual way to connect equipment. Might cause distortion. If you want to improve your sound system, I still recommend solve it from the source.
If you really like colorfly clean sound signature, next time when you save up can go straight for c4. It's a really beautiful sounding dap. Build in amp quite strong, clean, good punch and crystal clear. I have chance to audition once in China.
Else lower budget, can consider hifiman hm700. It's selling dap alone 16gb version and dap+ iem version. With balance output, the hifiman iem sounds even more open, cleaner with better details. Hifiman unique signature is like a speaker, flexible and comfortable, not very punchy and more on the mids.
Or you can also consider getting portable dac/amp to connect to your handphone but make sure your handphone model is otg supported. I would suggest you to go for e17. E18 is closer to x3 sound signature, warmer. E17 is cleaner sound though details is slightly inferior than e18.
much help.